Why does aluminum melt need degassing before casting?

Why is there gas in metal melt? What is the source of gas in copper or aluminum melt?

For all casting factories, especially for high level foundries, purity of the metal melt is a very important factor to be considered. In the past, people had been conducting researches on finding more efficient and economic ways to eliminate hydrogen, oxide and alkaline substances in metal melts. Those efforts are all for improving the quality of casts, as well as higher casing efficiency and lower costs, so that to raise product competitiveness.

The major way to raise efficiency is to reduce the quantity of soluble hydrogen in the AL alloy. Hydrogen is the only gas with a certain solubility to the AL melt. It is the product of chemical reaction of AL melt and moisture under melting. Moisture comes from the following sources:

1. Moisture from the air.

2. Yieldings of the burning fuel.

3. Moisture attached to loaded materials.

4. Remained grease on the waste.

When waste is soaked in under melting temperature, both chemical reaction of the grease and metal melt and crack of the waste will cause release of hydrogen.

5. Moisture attached to the in-furnace tools

Tools for feeding, mixing and waste removal will natural be attached by moisture.

6. Moisture attached to solid refining flux or residue, or chemically created.

Chlorides of aluminum and magnesium are the side-products of using chlorine as refining flux. Those chlorides can easily absorb water and attach to tools or furnace internal face.

7. Halfway dry wall of the container.

Furnaces, furnace lining and launders, which are made with fire clay coating and fireproofing materials, are not utterly dry before making contact with the AL melt. This will effect on the generating of hydrogen.  

8. Gases

Refining fluxes into the AL melt for various purposes may contain moisture. If insisting on some methods to reduce moisture into the melt, the hydrogen content can be lowered down. Besides, the inside temperature of the furnace should be strictly controlled, for hydrogen solubility and oxides forming will increase as temperature rises. If possible, max temperature ought to be kept under 760℃.

During melting, except for solubility, gas may enter the melt due to AL melt reacting with air moisture in the process of Al melt transfer or casing, especially when oxidation film of the melt surface is broken by tumbling. The solubility of hydrogen is slower when the melt surface is still.